In the case where a transistor is formed on a semiconductor wafer, electrodes of the transistor are formed on the wafer. In this case, each contact between the semiconductor wafer and each electrode is required to be an ohmic contact which has a small contact resistance. When, for example, a bipolar transistor is formed on a III-V compound semiconductor wafer using Ti/Pt/Au known as standard electrode material so as to form emitter electrodes or the like of the transistor, a problem described below arises. Since Ti in the electrode material is thermally unstable, it is difficult to form excellent ohmic electrodes on a region of the III-V compound semiconductor.
Therefore, conventionally used for forming ohmic electrodes on a III-V compound semiconductor wafer is thermally stable material with a high melting point, such as a metal nitride having a high melting point, or a metal silicide having a high melting point. Given as examples for the former are tungsten nitride (WN), molybdenum nitride (MoN), and titanium nitride (TiN). Given as examples for the latter are tungsten silicide (WSi), molybdenum silicide (MoSi), and titanium silicide (TiSi)
FIGS. 2(a) through 2(c) are process drawings illustrating a conventional method of forming electrodes. The conventional method of forming electrodes will be described below, with reference to these figures.
First of all, a resist layer 23 is formed on a III-V compound semiconductor wafer 21 having an insulating layer 22 on the surface (see FIG. 2(a)).
At the next stage, apertures 24 are formed at predetermined regions in the resist layer 23 which correspond to regions where electrodes are formed. Thereafter, etching is carried out to the insulating layer 22 utilizing the resist layer 23 as mask, so that a contact hole 25 is formed in the insulating layer 22 (see FIG. 2(b)).
Then, electrode material (for example, WN) is accumulated on the wafer 21, with the resist layer 23 unremoved. Thereafter, the resist layer 23 is removed, thereby completing formation of electrodes 26 at the predetermined regions (see FIG. (c)).
However, an ohmic contact failure sometimes occurs to the electrode 26 thus formed by the conventional method, which leads to an increase in a contact resistance (the contact resistance exceeds 1.times.4.sup.-4 .OMEGA..cm.sup.2, while a normal contact resistance is 3.times.10.sup.-7 .OMEGA..cm.sup.2). The ohmic failure causes a decrease in a yield of the semiconductors in the semiconductor manufacturing process.